Endostyle

The endostyle is an organ found in invertebrate chordate species of tunicates and lancelets, and in the larval stage of vertebrate lampreys. It assists in filter-feeding.[1][2][3] It has evolved into the thyroid in vertebrate chordates.

Since the endostyle is found in all three chordate lineages, it is presumed to have arisen in the common ancestor of these taxa, along with a shift to internal feeding for extracting suspended food particles from the water.[4] When feeding, food particles suspended in the water adhere to the mucus the endostyle produces.[5] The filtered water is then expelled through the gill slits, while the food and mucus are swept into the esophagus by movements of the cilia that coat the endostyle.[5][6]

The endostyle of larval lampreys (ammocoetes) metamorphoses into the thyroid gland in adults, and is regarded as being homologous to the thyroid in other vertebrates due to its iodine-concentrating activity.[7]

One early hypothesis for the function of the endostyle, developed in 1873 by Muller, proposed that the ammocoete endostyle has extremely similar functions as the tunicate hypobranchial groove. Numerous investigations into the endostyle ensued, only for the theory to be denied by future researchers. However, during this research, it was found that ammocoete endostyles can accumulate radioactive iodine isotopes. This revived academic interest in the endostyle. Already in 1963, research had concluded that cephalochordate and tunicate endostyles have the ability to capture iodine, thus further perpetuating new research.[6] A half century later, the homology between the thyroid in vertebrates and the endostyle in amphioxi and ascidian larvae was further supported by showing that their development involved fairly homologous transcription factors.[8] Similar genetic studies on a hemichordate[9] tentatively indicate that the endostyle also might share an origin with the stomochord.[9]

  1. ^ Luttrell, Shawn M.; Swalla, Billie J. (2015). "Genomic and Evolutionary Insights into Chordate Origins". Principles of Developmental Genetics. pp. 115–128. doi:10.1016/B978-0-12-405945-0.00007-7. ISBN 978-0-12-405945-0.
  2. ^ Satoh, Noriyuki (2016). "The New Organizers Hypothesis for Chordate Origins". Chordate Origins and Evolution. pp. 97–120. doi:10.1016/B978-0-12-802996-1.00007-9. ISBN 978-0-12-802996-1.
  3. ^ Satoh, Noriyuki (2016). "Deuterostomes and Chordates". Chordate Origins and Evolution. pp. 1–16. doi:10.1016/B978-0-12-802996-1.00001-8. ISBN 978-0-12-802996-1.
  4. ^ Maenhaut, C.; Christophe, D.; Vassart, Gilbert; Dumont, Jacques; Roger, P. P.; Opitz, Robert (2000). "Ontogeny, Anatomy, Metabolism and Physiology of the Thyroid". Endotext. MDText.com, Inc. PMID 25905409.
  5. ^ a b Jordan, E. L.; Verma, P. S. (2010). Chordate Zoology. S. Chand Publishing. ISBN 978-81-219-1639-4. OCLC 712010960.
  6. ^ a b Olsson, Ragnar (September 1963). "Endostyles and endostylar secretions: A comparative histochemical study". Acta Zoologica. 44 (3): 299–328. doi:10.1111/j.1463-6395.1963.tb00411.x.
  7. ^ Ogasawara, Michio; Di Lauro, Roberto; Satoh, Nori (June 1999). "Ascidian Homologs of Mammalian Thyroid Transcription Factor-1 Gene Are Expressed in the Endostyle". Zoological Science. 16 (3): 559–565. doi:10.2108/zsj.16.559. hdl:2433/57227.
  8. ^ Hiruta, Jin; Mazet, Francoise; Yasui, Kinya; Zhang, Peijun; Ogasawara, Michio (July 2005). "Comparative expression analysis of transcription factor genes in the endostyle of invertebrate chordates". Developmental Dynamics. 233 (3): 1031–1037. doi:10.1002/dvdy.20401. PMID 15861404.
  9. ^ a b Fritzenwanker, Jens H; Gerhart, John; Freeman, Robert M; Lowe, Christopher J (December 2014). "The Fox/Forkhead transcription factor family of the hemichordate Saccoglossus kowalevskii". EvoDevo. 5 (1): 17. doi:10.1186/2041-9139-5-17. PMC 4077281. PMID 24987514.
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